A fuel cell is a device which uses an electrochemical reaction to convert chemical energy stored in a fuel such as hydrogen or methane into electrical energy. In general, fuel cells include an anode to catalytically react with the fuel and a cathode in fluid communication with an oxidant such as air.
Fuel cells are typically arranged in a stacked relationship. A fuel cell stack includes many individual cells positioned between a fixed end plate and a free end plate. One fuel cell stack configuration includes an externally manifolded stack, wherein the fuel cell stack is left open on its sides and a fluid such as a fuel or oxidant is delivered by way of manifolds sealed to peripheral portions of respective sides of the fuel cell stack. The manifolds thus provide sealed passages for delivering the fuel and the oxidant gases to the fuel cells and directing the flow of such gases in the stack, thereby preventing those gases from leaking either to the environment or to the other manifolds. Such manifolds are typically used in Molten Carbonate Fuel Cells (MCFC) which operate at approximately 650° C. During operation of MCFCs, the fuel cells can move relative to the end plates.
Conventional fuel cells typically include an anode and a cathode separated by an electrolyte contained in an electrolyte matrix. The anode, the cathode, the electrolyte and the electrolyte matrix are disposed between a first collector and a second collector, with the first collector adjacent to the anode and the second collector adjacent to the cathode. Fuel flows to the anode via the first collector and an oxidant flows to the cathode via the second collector. The fuel cell oxidizes the fuel in an electrochemical reaction which releases a flow of electrons between the anode and cathode, thereby converting chemical energy into electrical energy.
The fuel cells described above can be stacked in series with separator plates disposed between adjacent fuel cells and end plates (e.g., a fixed end plate and a free end plate) disposed on opposing ends of the fuel cell stack. Fuel cells are stacked to increase the electrical energy they produce. Fuel cell stacks have a negative side with a negative end cell and a positive side with a positive end cell.
Ceramic materials such as sheets and fabrics comprised of ceria (CeO2), zirconia (ZrO2) and alumina (Al2O3) have been used in high temperature sealing and refractory applications. In particular, such ceramic materials have been used to manufacture conventional gaskets for wet and/or dry sealing of various high temperature fluids. However, such conventional gaskets are pliable and tend to sag when handled or otherwise manipulated during an assembly process. In addition, conventional gaskets have relatively low compressive strength. For example, conventional gaskets can irreversibly crush and achieve a strain of almost 0.9 in/in, when subject to a relatively low compressive load, thereby degrading the gaskets' sealing performance.
Gaskets made from ceramic materials have also been used in various fuel cell applications. A problem sometimes experienced by liquid phase fuel cells such as MCFCs is electrolyte migration which is characterized by the loss of the electrolyte from the positive end cell and the gain of electrolyte by the negative end cell. Electrolyte migration is caused by an electric voltage gradient along the length of the stack and generated by the cells in the MCFC stack. Loss of the electrolyte from the positive end cells can cause gas pockets in the electrolyte matrix of the positive end cell. This results in an irreversible increase in internal electrical resistance causing a significant voltage drop across the positive end cell and therefore decreasing the useful life of the MCFC stack. Migration of the electrolyte towards the negative end cell can also cause the negative end cell to become flooded with electrolyte, thereby reducing MCFC stack performance and life.
Electrolyte migration can occur because the electrolyte is a molten liquid when the MCFC is at its operating temperature, for example, as discussed in U.S. Pat. Nos. 8,088,697 and 7,294,427. Thus during operation, when the electrolyte is liquid, the electrolyte can flow along an outer surface of the MCFC stack. In particular, the electrolyte can flow in and/or under a gasket disposed between the outer surface and a manifold used to channel fluid such as fuel and air to the fuel cell. Problems with conventional ceramic gaskets (e.g., felts of zirconia, alumina and ceria) used for liquid phase fuel cells include: absorbing high amounts of electrolyte, acting as a conduit for electrolyte movement and having low strength. The low strength of these materials makes them difficult to handle and install in fuel cells. Ceramic gaskets such as those consisting of ZYF100 zirconia felt manufactured and as received from by Zircar Zirconia, Inc. of Florida, N.Y. have been used as a material for MCFC gaskets. However, such gaskets typically cause performance problems associated with electrolyte migration and have poor mechanical properties (e.g., low compressive strength and significant sag).
Attempts have also been made to identify ceramic gasket materials with reduced electrolyte absorption to reduce electrolyte migration in MCFCs. However, the conventional zirconia and alumina gasket materials have high electrolyte absorption and undesirable migration rates. Furthermore, alumina is shown to be unstable and reacts with molten alkali carbonate electrolyte to form wettable LiAlO2.